A nuclear reactor of the above-mentioned kind is disclosed in Canadian Patent Specification No. 1,070,860. In this known reactor, the flow-driving tendency of the shutdown circuit is constituted to a predominant extent by a gas cushion included in the shutdown circuit in the upper part of the reactor vessel. The pressure difference which, during normal operation of the reactor, acts on the shutdown circuit is substantially equal to the static pressure difference between two pool liquid levels, the spacing apart of which is equal to the vertical dimension of the gas cushion. During normal operation of the reactor this pressure difference is substantially equal to the pressure drop across the reactor core. The imbalance required to enable the pressure difference of the shutdown circuit to drive a flow of pool liquid through the core will only occur if a significant change of the lifting pressure of the circulating pump takes place. A reaction to too high a temperature in the reactor cooling water can only be obtained indirectly, namely in the form of a reaction to an increase in hydraulic resistance occurring in the primary circuit due to the production of bubbles of steam. Such an increase in the hydraulic resistance in the primary circuit will reduce the cooling water flow through the core, and thus the pressure drop across the core will be reduced. A condition for sufficient reactor safety is then that a sufficiently large volume of steam bubbles, with a good safety margin, is achieved before the water in the core starts boiling. Since the steam volume in each bubble is reduced with increasing pressure, this condition sets a relatively low limit on the operating pressure at which this known reactor can be used. Calculations have shown that this pressure should be below 10 bar, which means that this known reactor is not well suited for generating steam for steam turbines.
Furthermore, it is a disadvantage in this known reactor that the driving pressure difference, which upon an emergency shutdown, for example due to a cut-out of the circulating pump, acts in the shutdown circuit, is increasingly reduced as when the amount of pool liquid supplied to the reactor core increases, and is equal to zero when the amount of pool liquid supplied to the reactor vessel corresponds to the volume of the gas cushion. This means that a predominant part of the flow-driving tendency, which is inherent in the shutdown circuit but latent during normal reactor operation, is not capable of driving liquid through the core for any appreciable part of the time during which the decay power of the core is still relatively great, for example greater than 50% of the maximum decay power.